1. Field of the Invention
The invention pertains to transverse RF excitation gas lasers with circular geometry having external electrodes and to the attachment of mirrors to the laser discharge tube.
2. Prior Art
Since their earliest development, gas lasers have been constructed using hollow dielectric tubes. Excitation of the active laser gaseous medium has been traditionally accomplished by applying a relatively large DC voltage longitudinally along the length of the discharge tube via two or more metal electrodes placed in contact with the gaseous medium at the ends of the discharge tube or at points intermediate to the ends. Early in the development of gas laser technology, the advantages of removing metal electrodes from contact with the active gaseous laser medium and using RF excitation was recognized. However, interest in removing the metal electrodes from contact with the active gaseous laser medium apparently did not reach the peak necessary to lead to the development of a laser utilizing the technique. In a similar manner, RF excited lasers were left to future development.
Recent development of waveguide lasers has stimulated renewed interest in RF laser excitation, and particularly inductive RF coupling to the laser as disclosed in U.S. Pat. No. 3,772,611 issued Nov. 13, 1973 to Peter W. Smith. The inductive coupling mechanism disclosed by Smith was ineffective in providing for high frequency excitation and results in a non-uniform discharge.
U.S. Pat. No. 4,169,251 issued Sept. 25, 1979 to Katherine D. Laakmann discloses a method for obtaining transverse RF discharge excitation of a waveguide laser. This method requires contact between the laser medium and the transverse metal electrode structure. Problems with reactions of the excited gas with the metal electrodes inside the laser discharge tube will inevitably lead to reduced laser lifetime in a sealed laser and ultimate degradation of laser performance. The invention disclosed in the Laakmann patent required a generally rectangular laser geometry rather than circular. This results in the probability of excitation of undesirable optical modes rather than the axially symmetric modes characteristic of a circular geometry. Furthermore, the Laakmann lasing device specifically is limited to transverse RF excited waveguide lasers.
With the metal electrodes in contact with the discharge medium, random discharge instabilities can occur in the discharge medium resulting in fluctuating laser output power as well as mode instability. Further, the rectangular "slab" construction of the waveguide geometry makes it virtually impossible to apply mirrors directly to the ends of the laser structure. The application of mirrors directly to the ends of the laser is highly desirable for modular construction and long laser lifetime.